The present invention relates to a digital multi-frequency receiver, in particular, relates to such an apparatus which is utilized for the detection of tone signals in a digitalized telephone switching system.
The present invention is utilized preferably for the detection of MF (Multi-Frequency) signals in a trunk line between telephone exchange stations when said MF signals are in the No. 5 standard (recommendation Q.213) recommended by CCITT (International Telegraph and Telephoe Consultative Committee) and CCITT is one of the subsidiary organizations of the United Nations.
According to that No. 5 system, there are six frequency signals (MF signal), 700 Hz, 900 Hz, 1100 Hz, 1300 Hz, 1500 Hz, and 1700 Hz. The combination of two frequencies of those six frequencies are transmitted at the same time for controlling telephone exchange systems and/or subscriber terminals. The No. 5 system also defines the level of the MF signals, that is to say, the level of those signals must be in the range higher than -26 dB and must be lower than -4 dB. When the level is lower than -36 dB, that signal must be neglected. When the level is in the range between -36 dB and -26 dB, it is the discretion of the reception side whether the signal is detected or not. Further, the levels of the two frequencies received at the same time may have the level difference less than 7 dB.
When the MF signals are in an analog form, the detection of each of the MF signals are performed through a plurality of analog type bandpass filters. However, when the MF signals are in digital form, the MF signals must be detected through a digital process.
A prior multi-frequency detection system in a digital form utilized the principle of the Discrete Fourier Transform (DET) process. FIG. 1 is the block diagram of the prior frequency detection system utilizing that DFT process.
In FIG. 1, the reference numeral 1 is an input terminal receiving an input signal involving MF signals in a digital form, 2 is a multiplicator, 3 is a window function generator for providing the predetermined time slot (for instance 10 mS) with the predetermined amplitude characteristics for the DFT process, 4 is a DFT calculator, 5 is a sin signal generator which provides the sinusoidal wave having the same frequency to be detected, 6 is a cos signal generator which provides the cosine signal having the same frequency to be detected. The reference numeral 7 is a reference level source, 8 is a comparator, 9 is a latch and majority decision circuit, and 10 is an output terminal providing the detected MF signals.
In FIG. 1, the input signal applied to the input terminal 1 is applied to the multiplicator 2 which provides the product of the input signal and the window function. The output product of the multiplicator 2 is applied to the DFT circuit 4, which also receives the core frequency f.sub.k from the generators 5 and 6. The calculation in the DFT circuit is the following formula; EQU (.SIGMA.f.sub.i sin 2.pi.f.sub.k t).sup.2 +(.SIGMA.f.sub.i cos 2.pi.f.sub.k t).sup.2
where f.sub.i is the input signal frequency from the input terminal 1, sin 2.pi.f.sub.k t is the output of the generator 5, and cos 2.pi.f.sub.k t is the output of the generator 6. The comparator 8 compares the output of the DFT circuit 4 with the reference level provided by the reference level source 7, and the output of the comparator 8 is applied to the output logic 9 which holds the comparator output and performs the majority decision to determine the output signal. The output of the circuit 9 is applied to the output terminal 10 providing the detected MF signal.
In the above circuit, the DFT circuit 4 has the characteristics equivalent to a bandpass filter having the center frequency f.sub.k.
The DFT circuit has the natures as follows.
(1) The DFT circuit can detect only the frequency f.sub.k (=i/T.sub.w), where i is an integer from 1 to t.sub.w /2T, T.sub.2 is the period of the window function, T is the sampling period of the input signal. Accordingly, the period of the window function must be equal to the greatest common measure (GMC) of all the frequencies to be detected. When the MF signals with 700, 900, 1100, 1300, 1500 and 1700 Hz are to be detected, that period must be 10 mS (100 Hz).
(2) The characteristics of the bandpass filter of the DFT circuit is determined by the duration and the curve of the window function. Therefore, the duration of the window function restricts the high speed calculation in the DFT circuit.
(3) Since the input signal is not synchronized with the window function, the characteristics of the bandpass filter by the DFT circuit is deteriorated when the input signal starts or stops during the window function period and/or the input signal is interrupted for a short time during the window function period.
Accordingly, the prior frequency receiver based upon a DFT circuit has the disadvantages that it takes a long time to detect the frequency when the duration of the window function is long due to the small greatest common measure of the frequencies to be detected, and that the allowable level range of the input signal is rather narrow, since that range is defined by the window function, and the reference level is fixed.